Optical fiber amplifiers are used in telecommunications systems to boost the power of an optical signal derived from a modulated laser diode or other laser signal source prior to transmitting the signal along a fiberoptic waveguide. Fiber amplifiers are also used as relay elements to restore the power of the optical signal lost to absorption or scattering while being transmitted along a length of the fiberoptic waveguide. In cable television systems, in which an optical signal is sent to multiple receiving stations, fiber amplifiers restore the power of the signal at each splitting junction, as described, for example, in U.S. Pat. No. 5,331,449 to Huber et al. In fiberoptic telephone systems, wave-length-division multiplexing may be used, as described, for example, in U.S. Pat. No. 5,337,175 to Ohnsorge et al. and U.S. Pat. No. 5,339,183, to send multiple independent optical signals of slightly different wavelengths along the same optical fiber and to both send and receive signals on the same optical fiber in a two-way transmission scheme. Fiber amplifiers are used in such systems both to amplify the outgoing optical signal and to reamplify the incoming signal. Various forms of laser pulse modulation have been used in these fiberoptic systems.
Lasers have also been used in printer applications, where a modulated laser beam scans the photosensitive surface of a drum to produce a charge image of the page to be printed. It would be very desirable to be able to use optical fiber amplifiers with a laser diode signal source and a laser diode pump source in such printer systems. The laser diode signal source is capable of providing a 10 mW to 100 mW, single mode modulated light beam at very high modulation rates (typically greater than 10 MHz) with very low modulated drive currents, which can then be amplified to higher powers (greater than 1 W) by the fiber amplifier. However, one problem with using such a fiber amplifier is that the signal power levels typically deplete the excited state of the dopant ions in the fiber amplifying medium in a very short time compared to normal excited state life-times, so that the population inversion, and consequently the gain, within the fiber amplifier at any given time will depend to a great extent on the recent past history of the signal beam. If the signal source has recently been `on` for a while, the fiber amplifier will have a smaller population inversion than if the signal source has just been `off` for an extended time. The gain for the following signal pulses will then be lower. As a result, pulses following an extended on time have amplified intensities which are lower than pulses following an extended off time. This creates added noise in a communications system which may or may not be tolerated, depending on the type of signal modulation. In a printer system, the intensity variations can create noticeable grey scale errors in the printed image. Extended `on` and `off` periods are common when printing graphical images. While less common when printing text images or in communications applications, any extended `on` and `off` periods can still cause errors when they do occur.
An object of the invention is to provide an optical amplifier modulation system, especially adapted for use with optical fiber amplifiers, providing stable output pulse intensities independent of modulation history.